Initial Conditions Problem (pneumatics)

I'm trying to simulate a computer controlled pressure regulator. My problem is that I'm trying to use a constant volume block (to store a specific gas mixture), and I want to connect it (branch) to a piston block, and a variable area orifice block which exhausts to atmospheric reference. I keep getting initial condition errors when I connect it this way. The errors seem to be related to pressure. I have the pressure at the piston and chamber set equal, but something else is wrong. I'm sure it's some fundamental misunderstanding of the modeling technique.

I'd appreciate any thoughts.

Ron